This invention is related to displays which specularly and diffusely reflect light incident on the display surface back toward a frontal viewing space so as to manifest a desired indicia pattern. Such displays will be generally referred to herein merely as reflective displays.
One of the most common types of reflective display is the liquid crystal cell operated in the reflective mode. As is well known in the art, nematic liquid crystal material changes from a light transparent state to a light scattering state when subjected to an electric field. Thus by application of an electric field across patterned portions of a body of liquid crystal material an indicia pattern defined by the combination of transparent areas and light scattering areas is generated. When backed by a light reflective surface and illuminated from the front, the generated indicia pattern is manifested to an observer located in the frontal viewing space. However, the indicia pattern, or the "picture" produced, has very limited contrast. Moreover, the reflective surface backing the liquid crystal cell produces undesirable reflections of random objects, many times reflecting distracting movement or annoying light sources.
One approach to improving contrast is shown in U.S. Pat. No. 3,499,112, Heilmeier et al. Therein is described a flat, reflective type liquid crystal cell housed in a box behind an aperture approximately the same size as the cell. Hinged at one edge of the aperture, a black opaque lid is raised to permit observation of the cell at grazing angles over the opposite aperture edge. The lid protrudes significantly forward of the cell and is angularly positioned to intercept optical ray paths between itself and the observer which are specularly reflected from the reflective surface of the cell. Thus the transparent areas appear darker than the light diffusing areas.
Such an approach has several limitations. The enclosure plus protruding lid produces a very limited frontal viewing angle from which the indicia pattern may be viewed in total. Moreover the significant forward protrusion of the lid is undesirable in many applications such as aircraft cockpit displays and electronic watch displays. The same is true of other applications where thin substantially flat displays are desired or a substantially flush mounting is desired. Also, the protrusion of the lid prevents a great deal of ambient light from impinging on the diffusing areas of the liquid crystal, thereby reducing brightness of these areas. Such a reduction in brightness may counteract at least part of the additional contrast provided by the black protruding lid. Furthermore, as will be later seen, the lid blocks the light which would be most advantageous for illumination.
Thus it is an object of this invention to provide an improved reflective display apparatus which overcomes these and other disadvantages of the prior art.